The use of an electrical plasma for chemical conversion of ingredients—like NOx (NO, NO2, . . . ), coal, soot etc.—of exhaust gases is known from various (patent) publications since about 1994.
In presence of water and/or oxygen the plasma produces strongly oxidizing radicals such as OH, HO2, O and negative ions of O2. It has also been shown by chemical kinetics calculations (R. Dorai, J. Appl. Phys. Vol 88, No. 10, 2000) that electron impact dissociation of hydrocarbons in exhaust gases contributes to the total concentration of oxidizing compounds.
In order to apply plasma's for soot and/or NOx conversion, gas permeable dielectric filter materials have been proposed in an interelectrode gap. Examples of these materials are packed pellet beds and ceramic foams with a variety of main pore sizes. These materials are essential for either collection of materials such as soot particles, dust particles or particles containing living micro-organisms (the collection may be mechanical or electrostatically enhanced) or because of their catalytic properties.
A number of attempts have been made to convert both soot particles and NOx in a single plasma reactor. However, because in electrical plasma's NO is readily oxidized to NO2, and soot oxidation by NO2 reproduces NO, segmented approaches have been investigated too. According to the segmented approaches, soot is filtered and converted by plasma in a first segment, and the remaining NOx concentration is further treated by plasma in a second segment, preferentially in presence of a catalyst.
For soot conversion, different filter and electrode configurations have been proposed. In wall-flow type plasma filters, e.g. proposed by S. Müller et al. (Hakone Conference, Greifswald, 11-13 Sep. 2000), the filter pore size is sufficiently small to collect the main particle mass at the filter surface. In the so called flow-through filter type, the particles are collected within a ceramic packed bed or within a relatively open ceramic foam where the main pore size is large compared to the particle size. A plasma filter of this type has been proposed by AEA Technology in patent WO00/71866.
For optimization of NOx conversion, plasma reactors have been proposed with relative large non-gas permeable electrodes, where at least one of the electrodes is covered by a dielectric barrier coated with catalytic material. Examples of this type of reactors are published in patents (e.g. US2001/0040089 of Delphi Technologies).
A drawback of existing wall-flow plasma filters is that either the plasma's cause smaller particles that are either not filtered or the filter pore size causes an unacceptable pressure drop. Flow-through plasma filters suffer from relatively large filter thickness requirements and inhomogeneous plasma distribution. An important drawback of existing plasma reactors in general, for both soot and NOx conversion, is their high energy consumption. For automotive applications an extra fuel consumption of 2-4% for generating the electrical plasma may be acceptable. However, the electrical plasma power at this given fuel consumption is unsufficient for effective conversion of soot and NOx using the known plasma reactors.